The present invention relates to a spring-type brake actuator for the braking system for a vehicle, and in particular to an internal venting/breathing system to minimize entry of potentially contaminated exterior environment air into such an actuator.
It is well known to employ so-called “spring brake” actuators to provide service, parking and emergency brake operation on vehicles such as commercial trucks, tractors and trailers equipped with lever-operated drum or disc brakes. Spring-type brake actuators are typically pneumatically operated, and are supplied with operating air from a compressed air source on the vehicle. These actuators also typically are arranged in a “fail-safe” manner, i.e., where the actuator defaults to a brake application state upon loss of operating air pressure.
An example prior art spring brake actuator is shown in cross-section view in FIG. 1. Actuator housing 1 includes a rear cylinder 2 in which a rear piston 3 is displaceably arranged. The inner wall of the rear cylinder and a chamber-side of the rear piston define a rear ventilation chamber 4. The other side of the rear piston bears on a brake actuator spring 5. This spring is also known in the art as a “power spring” or a “parking brake spring,” and these terms may be used interchangeably.
The rear ventilation chamber is isolated from the spring side of piston 3 by an annular seal 6. An intermediate flange 8 (also known as a “wall”) separates rear cylinder 2 from a front cylinder 9. The intermediate flange 8 traversed by a seal 10 through which passes a parking brake application rod 11, formed as an extension of rear piston 3. The parking brake application rod 11 can be displaced in the intermediate flange 8 by the rear piston. A front ventilation chamber 7 within front cylinder 9 is delimited by the cylinder inner wall and a front piston 13 and annular diaphragm 14. The rear piston 3 and the front piston 13 are in non-coupled contact with one another by means of the parking brake application rod 11, such that the front piston 13 can be displaced in a brake application direction by the rear piston 3 and/or by the application of pneumatic pressure in front ventilation chamber 7. An actuating rod 15 for actuating a brake lever of a vehicle brake is provided on the front side of the front piston 13.
FIG. 1 also shows mounting studs 16 provided for mounting of the actuator 1 on the vehicle brake, as well as a light return spring 18 which biases front piston 13 toward the rear of front chamber 7.
When no pneumatic pressure is present in the FIG. 1 actuator unit, the brake actuation spring 5 applies a high spring force to rear piston 3, which in turn applies this force via parking brake application rod 11 to front piston 13 to cause the actuator rod 15 to apply the vehicle brake. In this state, the vehicle brake functions as a parking brake, preventing vehicle movement.
When release of the parking brake is desired, the rear ventilation chamber 4 is filled with compressed air via a ventilation port (not illustrated). As the force generated by the increasing air pressure on the front side of rear piston 3 exceeds the force generated by brake application spring 5, the rear piston 3 and parking brake application rod 11 move toward the rear of the rear cylinder 2, compressing spring 5 and causing air in the rear of rear cylinder 2 to be vented to atmosphere through passages in rear piston 3 (not illustrated) to vent path 19 or side holes 16.
As parking brake application rod 11 moves towards the rear, the force previously applied to front piston 13 is relieved, and the return spring 18 biases the front piston 13 toward the rear of front cylinder 9, thereby withdrawing actuating rod 15 away from and releasing the vehicle brake. The vehicle therefore moves from a state in which it is braked by the brake actuator spring 5, to a non-braked state in which the vehicle may be moved. The vehicle brake is applied as a service during normal operation by admitting compressed air into the front ventilation chamber 7 (via a port not shown in FIG. 1). Because air pressure in rear ventilation chamber 4 continues to hold parking brake application rod 11 at the rear of the rear cylinder 2, the front piston 13 and actuating rod 15 are free to move forward and backward within the front cylinder as necessary to respond to the operator's brake actuation demands.
Existing designs which have the rear chamber containing the brake actuator spring vented to the atmosphere are vulnerable to internal corrosion due to exposure to the environment, where water and/or solid debris (“grit” or de-icing solids) may enter the chamber, causing the power spring and other components, including the housing itself, to corrode and potential suffer early functional failure over time. The venting to atmosphere is typically provided with at least one chamber breather on the spring side of the rear cylinder. These breathers relieve any pressure leaking into the rear of the actuator housing from the rear chamber, as well as providing a path for air to pass as the air in the power spring side of the rear chamber is compressed when the parking brake piston compresses the power spring. The corrosion concern arises from splashing of contaminated water or road debris onto the actuator housing where in can enter the power spring chamber, and from the inflow of external air when the rear chamber is depressurized and the brake actuator spring expands back to its parking position. The air entering the spring side of the cylinder through the breather contains water in the form of humidity, and the material splashed onto the actuator, e.g., rain water, road salt and de-icing solutions, are also sources of corrosive chemicals.
In view of the foregoing problems with current spring-type brake actuator parking brake pistons and related actuator components, the present invention provides an improved parking brake chamber ventilation/breather system which allows the use of internally-vented chambers and thus eliminate the need to allow intake of external air into the power spring chamber and the associated corrosion problems.
In the present invention the parking brake release actuator is provided with an internal breather valve. The spring housing may be provided with a one-way vent from the spring housing to the atmosphere. The internal breather valve is preferably located in the parking brake release actuator piston adjacent to a passage from the parking brake release pressure chamber. The internal breather valve is closed when the parking brake release chamber is pressurized to a high enough level to compress the power spring and withdraw the parking brake operating rod and is open when the parking brake release chamber is fully depressurized. The internal breather valve is also arranged to be momentarily open when the pressure in the chamber containing the power spring is only slightly higher than the pressure in the parking brake release chamber so that only clean, dry air from the parking brake release pressure chamber enters the power spring chamber. Advantageously, the internal breather valve may be located in a threaded insert which is in turn affixed (for example by press-fitting or welding) to a parking brake actuator piston made up from a stamped piston. The internal breather valve may also be provided in other parking brake actuator piston designs, such as a cast aluminum piston.
Further, to prevent excessive pressure in the power spring chamber, the spring actuator is provided with an elastomeric plug device which seals off the spring housing opening for insertion of a manual parking brake piston withdrawal rod, where the plug device has a one-way valve which permits pressure to escape from the power spring chamber (for example, as pressure in the chamber increases when the parking brake release piston is moved to the fully-released brake position) while sealing against flow of air from the outside environment into the spring housing. One of ordinary skill will recognize that the plug need not be formed from an elastomeric material, and may be formed for a plurality of subparts.
With the present breathing/ventilation system, only clean, dry air from the vehicle's air preparation system reaches the power spring chamber, as there is no path for environmental contaminates to enter the spring housing. Accordingly, the working life of the spring-type brake actuator is significantly extended. Further, where previous attempts at providing internal breathing brake chambers have required expensive-to-manufacture designs that have not been easily adapted to high volume, low cost, simplified spring brake production, the present system provides a low cost approach to providing clean, dry make-up air in a manner which minimizes the potential for contamination from external air sources. In addition, by providing for venting of pressure from the power spring chamber at a pressure which need only be nominally above the external pressure, the present invention permits the parking brake to be released at low operating pressures when the vehicle is on a grade (as when a driver must use the service brake when releasing the parking brake to prevent roll back of the vehicle), as the parking brake release actuator does not have to also overcome high pressure in the power spring chamber.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.